consolidation of single and double layers ceramic
TRANSCRIPT
CONSOLIDATION OF SINGLE AND DOUBLE LAYERS CERAMIC
STRUCTURE
NUR FARHANI BINTI ISMAIL
A thesis submitted in partial fulfilment of the requirement for the award of the
Degree of Master of Mechanical Engineering
Faculty of Mechanical and Manufacturing Engineering
Universiti Tun Hussein Onn Malaysia
OCTOBER 2012
v
ABSTRACT
Basically, the performance of ceramic structure is essential for the development of
ceramic system or component. This performance can be controlled from the early
stage i.e preparation of raw material up till the final stage i.e. finishing steps. In this
work, ceramic processing parameters at intermediate stage (consolidation) were
considered to obtain good properties and structure of alumina ceramic. The
microstructure, mechanical and physical properties of the green and sintered alumina
ceramic pellets (single and double layer) were investigated. Some modification
during consolidation (drying and firing) which focuses on the particle characteristics,
drying techniques, additive and types of sintering were conducted. Three drying
techniques (room, oven and microwave) were performed to the single and double
ceramic alumina. Several factors which are identified as strong influence towards the
sintered body i.e. characteristics (particle size and particle size distribution), MgO
additive and sintering types (conventional and HIP technique) were selected as
controlled parameters during the sintering step. All samples were fabricated using
uniaxial press prior to the consolidation stage and were characterized using standard
procedures. Based on the observation and measurement, it was found that the
microwave drying technique produced a homogenous and uniform microstructure
compared to the other drying techniques for both green and sintered bodies. The
density value of 25 µm sintered body (Al2O3) with microwave drying technique
shows an increment value from 3.551 g/cm3 to 3.624 g/cm
3 compared to others
drying technique. The same phenomena were also observed with the addition of
MgO. In fact, the highest density value was obviously shown with alumina pellets
sintered under HIP with same drying technique. In general, the increased of density
value causes a reduction of porosity thus produce a good and hard sintered structure
for all sintered samples.
vi
ABSTRAK
Pada asasnya, prestasi struktur seramik adalah penting untuk pembangunan
komponen atau seramik sistem. Prestasi ini boleh dikawal dari peringkat awal iaitu
penyediaan bahan mentah sehingga peringkat akhir iaitu langkah penyudahan.
Dalam kajian ini, parameter pemprosesan seramik diperingkat pertengahan
(pengukuhan) dipertimbangkan untuk mendapatkan sifat-sifat dan struktur alumina
yang baik. Mikrostruktur, sifat fizikal dan mekanikal pelet alumina yang belum dan
telah disinter (lapisan tunggal dan dua lapisan) telah disiasat. Beberapa
pengubahsuaian semasa pengukuhan (pengeringan dan pembakaran) diberi tumpuan
ke atas ciri-ciri zarah, teknik pengeringan, bahan penambah, dan jenis persinteran
yang dijalankan. Tiga teknik pengeringan (bilik, oven dan mikrogelombang) telah
dijalankan keatas seramik alumina tunggal dan dua lapisan. Beberapa faktor telah
dikenalpasti mempunyai pengaruh yang kuat terhadap jasad yang telah disinter iaitu
sifat-sifatnya (saiz zarah dan taburan saiz zarah), bahan penambah MgO dan jenis
persinteran (teknik konvensional dan HIP) telah dipilih sebagai parameter kawalan
semasa peringkat sinteran. Semua sampel telah dibentuk menggunakan tekanan satu
paksi sebelum ke peringkat pengukuhan dan pencirian dilakukan mengikut langkah
piawaian. Berdasarkan pemerhatian dan pengukuran yang dijalankan, didapati
pengeringan menggunakan mikrogelombang menghasilkan mikrostruktur yang
homogen dan seragam keatas jasad yang belum dan telah disinter berbanding dengan
teknik pengeringan lain. Nilai ketumpatan bagi jasad 25 µm (Al2O3) yang
dikeringkan dengan teknik mikrogelombang menunjukkan peningkatan dari 3.55
g/cm3 kepada 3.624 g/cm
3 berbanding dengan teknik pengeringan yang lain.
Fenomena yang sama juga dikenalpasti dengan bahan tambahan MgO. Secara umum,
peningkatan nilai ketumpatan menyebabkan pengurangan keliangan maka akan
menghasilkan struktur yang baik dan kuat untuk sampel yang disinter.
vii
CONTENTS
TITLE i
DECLARATION ii
DEDICATION ii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
LIST OF TABLES xi
LIST OF FIGURES xiii
LIST OF UNIT ABBREVIATIONS xvii
LIST OF APPENDICES xviii
CHAPTER 1 INTRODUCTION 1
1.1 Research background 1
1.2 Problem statement 2
1.3 Aim 3
1.4 Objectives of study 3
1.5 Research Scopes 3
CHAPTER 2 LITERATURE REVIEW 5
2.1 Introduction of ceramic materials 5
2.1.1 Alumina (Al2O3) 7
2.1.2 Silica (SiO2) 8
2.2 Fabrication of ceramic component 9
viii
2.3 Drying on ceramic structure 12
2.3.1 Mechanism of drying in ceramic materials 12
2.3.2 Drying technique 14
2.4 Sintering of ceramic structure 18
2.4.1 Solid state sintering 19
2.4.2 Liquid phase sintering 23
2.4.3 Factors that affect the sintering process 25
CHAPTER 3 METHODOLOGY 30
3.1 Introduction 30
3.2 Starting Material characterizations 32
3.2.1 X-Ray diffraction (XRD) 32
3.2.2 Thermal gravimetric analysis (TGA) 32
3.2.3 Particle size analysis 32
3.3 Fabrication process 33
3.4 Drying process 36
3.5 Sintering process 37
3.6 Characterizations 38
3.6.1 Phase characterization 38
3.6.2 Microstructure characterization 39
3.6.3 Apparent porosity and bulk density 39
3.6.4 Linear shrinkage 41
3.6.5 Moisture removal 41
3.6.6 Hardness test 42
CHAPTER 4 RESULTS AND DISCUSSIONS 43
ix
4.1 Introduction 43
4.2 Raw material characterization 43
4.2.1 Thermal gravimetric analysis (TGA) 43
4.2.2 Particle Size Analysis (PSA) 45
4.2.2 Phase Analysis (XRD) 49
4.3 Effect of the drying technique for the ceramic green body 53
4.3.1 Microstructural analyses of green body 53
4.3.2 Porosity and Density 54
4.3.3 Shrinkage 55
4.3.4 Moisture removal (%) 56
4.4 Effect of the drying technique to the ceramic sintered body 57
4.4.1 Al2O3 single layer and double layer structure 58
4.4.2 Porosity and density 65
4.4.3 Shrinkage 67
4.4.4 Hardness property 69
4.5 Effect of MgO additive to the ceramic sintered structure 70
4.5.1 Microstructural analyses 71
4.5.2 Porosity and density 77
4.5.3 Shrinkage 80
4.5.4 Hardness property 81
4.6 Effect of HIP sintering technique to the sintered pellet 82
4.6.1 Comparison of HIP and conventional sintering
with different particle sizes 83
4.6.2 Comparison of HIP and conventional sintering
with different mode of drying 89
x
4.6.3 Comparison of HIP and conventional sintering
with addition of additive 92
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 96
5.1 Conclusions 96
5.2 Recommendations 97
REFERENCES 98
APPENDICES 104
LIST OF ACHIEVEMENTS 116
VITA 117
xi
LIST OF TABLES
2.1 Additives used in Powder Pressing 11
2.2 Comparison of energy savings for conventional and microwave
drying and firing of ceramics [40] 17
2.3 Selected ceramic composition and the sintering process used during
densification [28] 19
2.4 Example of additive (dopant) for grain growth control in some
ceramic materials [4] 27
3.1 Particle size group 33
3.2 Formulation for fabrication ceramic pellet 34
3.3 Duration for each drying process. 37
4.1 Particle size group 45
4.2 Particle size distribution for Al2O3 powder 46
4.3 The results of the porosity (%) for the green body of ceramic
type (slurry and pellet) dried under room, oven and microwave 55
4.4 The results of the density (g/cm3) for the green body of ceramic
type (slurry and pellet) dried under room, oven and microwave 55
4.5 Linear shrinkage (%) for green body of ceramic type (slurry and
pellet) dried under room, oven and microwave 56
4.6 Moisture removal of ceramic green body 57
4.7 Hardness (Hv) measurement of Al2O3 sintered pellet (with and
without MgO additive) from additive different particle size against
method of drying. 82
4.8 Average grain size (µm) of Al2O3 pellet sintered with different
sintering technique from different particle size. 84
xii
4.9 Average grain size (µm) of Al2O3 pellet sintered with different
sintering technique from different drying technique. 90
4.10 The results of the porosity and density of Al2O3 pellet sintered with
different sintering technique with from different drying technique 90
4.11 Hardness (Hv) measurement of Al2O3 pellet sintered with different
sintering technique with from different drying technique 92
4.12 Average grain size (µm) of Al2O3, (25 µm) pellet (with and without
MgO additive) sintered with different sintering technique. 93
4.13 The results of the porosity and density of Al2O3 (25 µm) pellet (with
and without MgO additive) sintered with different sintering technique 93
4.14 Hardness (Hv) measurement of Al2O3 (25 µm) pellet (with and
without MgO additive) sintered with different sintering technique 95
xiii
LIST OF FIGURES
2.1 A comparison of different aspects of traditional and advanced
ceramics [14] 6
2.2 Relationships among the chemical term on silicon [3] 9
2.3 Processing of the ceramic structure [3] 9
2.4 Schematic for powder compaction process [3] 11
2.5 Schematic of the drying model [7] 13
2.6 Heating pattern in conventional and microwave furnaces [41] 17
2.7 Schematic of three stages in sintering [4] 20
2.8 Schematic of driving force for sintering [4] 21
2.9 Schematic of densification and coarsening process of microstructure
during firing process [4] 22
2.10 The surface of an Al2O3 ceramic during sintering [4, 28] 23
2.11 Schematic evolution of powder compaction during liquid phase
sintering [4] 24
2.12 Particle size distribution a) narrow and b) broader 26
2.13 Schematic for Hot Isostatic Pressing sintering 29
3.1 A schematic diagram showing the main processes used for the
fabrication of the ceramic structure in this work 31
3.2 Schematic representation of a single layer ceramic structure 34
3.3 Schematic representation of a double layer ceramic structure 34
3.4 Mixing process of Al2O3 and MgO with Planetary Mill Machine 35
3.5 Schematic die filling, compaction and ejection for powder
compaction 36
3.6 Sintering profile 38
3.7 Mass of the dry sample in air (Wd) 40
xiv
3.8 Mass of the specimen soaked in water (WS) 40
3.9 Indenter and indentation of Vickers Hardness [54] 42
4.1 Thermal gravimetric analysis, TGA of (a) weight loss (%) and
(b) deviation (mg/°C) for polyvinyl alcohol (PVA) mixed with Al2O3
powder 45
4.2 PSA for type A (25 µm) 47
4.3 PSA for type B (45 µm) 47
4.4 PSA for type C (63 µm) 48
4.5 PSA for type D (90 µm) 48
4.6 XRD analysis for alumina powder 49
4.7 XRD analysis for the silica powder 50
4.8 XRD analysis for the magnesium oxide powder 50
4.9 XRD pattern showing the comparison between the Al2O3 sintered
pellet and Al2O3 pellet with MgO 51
4.10 EDS analysis for the sintered Al2O3 pellet with MgO additive 52
4.11 SEM micrograph (2000× magnification) of ceramic green body with
different drying techniques, (a) room (b) oven and (c) microwave 53
4.12 SEM micrograph (1500× magnification) of the sintered Al2O3 pellet
(without MgO additive) with particle sizes (a) 25 µm , (b) 45 µm
(c) 63 µm and (d) 90 µm from different drying technique; room,
oven and microwave 58
4.13 Schematic of grain size measurement 60
4.14 Average grain size (µm) for different particle sizes of sintered
Al2O3 pellet from different drying technique (room, oven and
microwave 61
4.15 Cross sectional images (100× magnification) of sintered double layer
ceramic structure (Al2O3 / Al2O3) from different drying (a) room,
(b) oven and (c) microwave 62
xv
4.16 Cross sectional images (100× magnification) of sintered multilayered
ceramic structure (Al2O3 / SiO2) from different drying (a) room,
(b) oven and (c) microwave 63
4.17 The results of porosity (%) for sintered Al2O3 pellet with different
particle size type dried from different drying technique 66
4.18 The results of density (g/cm3) for sintered Al2O3 pellet with different
particle size type dried from different drying technique 66
4.19 Shrinkage (%) (a) top and (b) bottom layer for sintered ceramic pellet
from different particle sizes against method of drying 68
4.20 Hardness (Hv) measurement of Al2O3 sintered pellet from different
particle size against method of drying 70
4.21 SEM micrograph (1500× magnification) of the sintered Al2O3 pellet
with particle size 25 µm from different drying techniques (a) room,
(b) oven and (c) microwave 71
4.22 SEM micrograph (1500× magnification) of the sintered Al2O3 pellet
with particle size 45 µm from different drying techniques (a) room,
(b) oven and(c) microwave 72
4.23 SEM micrograph (1500× magnification) of the sintered Al2O3 pellet
with particle size 63µm from different drying techniques (a) room,
(b) oven and (c) microwave 73
4.24 SEM micrograph (1500× magnification) of the sintered Al2O3 pellet
with particle size 90 µm from different drying techniques (a) room,
(b) oven and (c) microwave 74
4.25 Average grain size (µm) of sintered Al2O3 pellet (with and without
MgO additive) with different particle size dried under different
drying technique 76
4.26 The results of the porosity (%) for sintered Al2O3 pellet (a) without
MgO additive and (b) Al2O3 pellet with MgO additive from different
particle size dried under different drying technique 78
xvi
4.27 The results of the density (g/cm3) for sintered Al2O3 pellet (a) without
MgO additive and (b) Al2O3 pellet with MgO additive from different
particle size dried under different drying technique 79
4.28 Shrinkage (%) (a) top and (b) bottom layer for sintered Al2O3 pellet
with MgO additive from different particle size against method of
drying 81
4.29 SEM micrograph (1500× magnification) of Al2O3 pellet sintered via
conventional and HIP technique from different particle (a) 25 µm,
(b) 45 µm, (c) 63 µm, and (d) 90 µm 83
4.30 The results of the porosity (%) for Al2O3 pellet sintered with different
sintering technique from different particle size 85
4.31 The results of the density (g/cm3) for Al2O3 pellet sintered with
different sintering technique from different particle size 85
4.32 Shrinkage (%) (a) top and (b) bottom layer for Al2O3 pellet with
different sintering technique from different particle size 87
4.33 Hardness (Hv) measurement of Al2O3 pellet sintered with different
sintering technique with from different particle size 88
4.34 SEM micrograph (1500× magnification) of Al2O3 (25 µm) pellet
sintered via conventional and HIP technique from drying technique
(a) room and (b) microwave 89
4.35 Shrinkage (top and bottom layer) for sintered Al2O3 pellet from
different drying technique against method of sintering technique 91
4.36 SEM micrograph (1500× magnification) of Al2O3 (25 µm) sintered
via conventional and HIP technique from drying technique (a) without
MgO additive and (b) with MgO additive 92
4.37 Shrinkage (top and bottom layer) for sintered Al2O3 (25 µm)
pellet (with and without MgO additive) against method of sintering
technique 94
xvii
LIST OF UNIT ABBREVIATIONS
Al2O3 Aluminium Oxide
SiO2 Silicon Oxide
TiO2 Titanium Oxide
Li2O3 Litrium Oxide
Y2O3 Ytrium Oxide
ZrO2 Zirconium Oxide
MgO Magnesium Oxide
ºC Degree Celcius
MW Microwave
HIP Hot Isostatic Press
XRD X-Ray Diffraction
PSA Particle Size Analyzer
TGA Thermal Gravimetric Analysis
PVA Polyvinyl Alcohol
EDS Energy Dispersive Spectroscopy
xviii
LIST OF APPENDICES
Appendix A Thermal gravimetric analysis (TGA)
Appendix B Particle size analysis (PSA)
Appendix C Effect of drying technique to the ceramic
sintered body
Appendix D Effect of MgO additive to the ceramic
sintered structure
Appendix E Effect HIP sintering to the sintered pellet
CHAPTER 1
CHAPTER 1 INTRODUCTION
1.1 Research background
Generally, the production of ceramic component needs a sequence step which ranges
from loose particles (powder) to the formation of hardened solid structure [3]. One of
the most important stages in this ceramic processing is the consolidation step which
consists of the drying and sintering stages [4, 5]. Controlling the processing
parameters or raw material formulation is very crucial to ensure the ceramic
properties and structure is in good condition. Basically, the drying process of the
ceramic materials is very crucial since it influences the physical, mechanical and
characteristics of the ceramic body [6]. Basically, the drying process is a complex
process involving simultaneous heat and mass transfer that will contribute to the loss
of moisture from the pore structure [7]. Theoretically, the movement of moisture (in
a ceramic body) is strongly related to the shrinkage mechanism, and this is a major
defect in ceramic product where this defect can range from the initiation of crack to
the failure of the fired body due to the crack propagation [8]
The next stage of ceramic consolidation step is sintering. Most dried ceramic
products must go through the sintering process in order to produce the dense product
with desired properties such as low porosity and high strength. Typically, the
sintering stage is the most critical step in ceramic processing because it involves the
complex mechanism in diffusion i.e., diffusion, mass transfer and grain growth etc.
Sometimes, the sintering aid (additive) is used during the sintering process to
promote sintering or controlling of the grain growth in producing good ceramic
structure [9, 10]. Furthermore, the sintering aids are often used in ceramic processing
in order to promote faster consolidation at low temperature especially for pure
2
alumina. Basically, pure alumina must be fired at high temperature in the range of
0.5 until 0.75 of the melting point [4] and it takes a long time to achieve high density.
Densification of the ceramic structure is not only influenced by the impurities
(dopant) but also influenced by the characteristic of the materials itself (i.e. particle
size, particle size distribution, particle shape, particle aggregate). One of the powder
characteristics is particle size distribution of the raw materials. Basically, narrow
particle size distribution promotes more densification [11, 12] as compared to
broader particle size. Besides that, the use of the pressure during sintering also can
help increase the densification rate. One of the most common methods used is HIP
where the pressure and inert gas were supplied during the process. This technique
has potential to produce high density with fine grain size for the pure ceramic.
As elaborated in detail in the previous paragraph, there are many factors that
influence ceramic sintered properties and structure. Most of the influence factors are
in consolidation (drying and firing). Therefore, in this investigation and observation
of these critical parameters or factors to final structure were considered.
The main emphasis of this study is to investigate the effect of drying heating
(room, oven and microwave), particle size (in range below 90 µm) and MgO additive
to the densification and microstructure of the Al2O3 pellet. The drying heating is also
investigated on the double layer pellet (Al2O3/Al2O3 and Al2O3 / SiO2) for better
understanding about the microstructure evolution. Analysis by utilizing scanning
electron microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and X-
Ray Diffraction (XRD) were carried out to investigate the microstructure, formation
of the SiO2 and MgO and phase change. The characterisation of the physical
(porosity, density and shrinkage) and mechanical properties (hardness) also were
performed because these are important properties for the ceramic structure.
1.2 Problem statement
Fabrication of the ceramic structure is very complicated because it involves water
removal and diffusion mechanism during the drying and firing (consolidation). In
general, properties and microstructure i.e. density, porosity, hardness and shrinkage
are strongly influenced by the drying and firing (consolidation) process. There are
many factors that influence the dried and sintered ceramic body i.e. powder
characteristics, drying technique, sintering technique and sintering aid. However, the
3
problem still arises when trying to develop ceramic structure during drying and
sintering, for example the densification is not easy to achieve for the ceramic body
structure, especially for hard Al2O3 ceramic particles. Therefore, further investigation
and observations need to be conducted to examine the effect of processing
parameters and factors to the consolidation process.
1.3 Aim
The main purpose of this study is to investigate the effect of the consolidation stage
(drying and sintering) with the influence of factors (powder characteristics, sintering
aid and sintering condition) towards properties, microstructure and morphology of
ceramic structure.
1.4 Objectives of study
The objectives of this study are:
i. To determine the effect of different drying to the Al2O3 ceramic structure
(slurry and pellet type).
ii. To determine the effect of different drying techniques towards the sintering
structure (single and double layer).
iii. To develop the sintered single Al2O3 structure via conventional and HIP
technique with controlled variable factors (particle size and additive )
1.5 Research Scopes
An understanding of the properties of the ceramic structure will lead to a deeper
understanding on the impact of its performance on the procedures that need to be
considered to develop good ceramic single and double layer structure. Therefore, the
scopes of this study have been outlined as follows:
i. Development of shell mould green body (slurry type) by using three drying
technique (room, oven and microwave).
4
ii. Preparation of the single Al2O3 (with and without MgO additive) structure and
double Al2O3 layer (Al2O3/ Al2O3 and Al2O3/SiO2) ceramic structures from
different particle sizes by powder (uniaxial) pressing.
iii. Development of single Al2O3 structures by using three drying techniques
(room, oven and microwave) and two sintering techniques (conventional and
HIP).
iv. Development of double Al2O3 layer structures by using three drying techniques
(room, oven and microwave).
v. Characterization of the phase analysis of Al2O3, SiO2 and MgO powder before
and after the consolidation process by using X-ray Diffraction Analysis (XRD).
vi. Observation of the microstructure of Al2O3 single and double layer structures
by using Scanning Electron Microscope (SEM).
vii. Characterization of the physical properties testing (porosity, density and
shrinkage) and mechanical properties (hardness) for Al2O3 structure.
CHAPTER 2
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction of ceramic materials
Ceramic materials are inorganic, nonmetallic materials which compose at least two
elements. Most of the ceramics are compounds between metallic and nonmetallic
elements which is bonded together primarily by ionic and/or covalent bonds.
Basically, most of the ceramic materials must go through a heat treatment process
(firing) at high temperature to produce the materials with desired properties. This
phenomenon is strongly related to the term of “ceramic” which comes from the
Greek word keramikos, which means “burnt stuff [13].
Ceramics can be divided into two groups, which is traditional ceramic and
advanced ceramics. Traditional ceramics include silicates such as clay, pottery,
bricks, porcelain and this group is still implemented until now in the industry as a
refractory due to high temperature resistance. Advanced ceramics is development of
the new ceramic product which have high temperature resistance, chemical,
mechanical characteristics and electrical goods in advanced technology. Basically,
advanced ceramic materials consist oxides such as alumina (Al2O3), silica (SiO2),
zirkonia (ZrO2) and barium titanate (BaTiO3) and non oxide group such as silicon
nitrade (Si3N4) , boron nitride (BN) and carbides [4].
Recently, ceramic materials have been widely used in the industry especially
in advanced technology such as refractories, spark plugs, dielectrics in capacitors,
sensors, abrasives, and magnetic recording media [4]. Most of the ceramic materials
are generally porous materials, brittle, high temperature and highly sensitive to the
physical thermal changes. Basically, properties of ceramic products are strongly
related to their chemical compositions and their atomic and micro scale structure [1].
Thus, controlling the materials and processing operation is needed to produce
6
product with good performance. Ceramic materials originally from powders and
must go through fabrication process to produce product.
Figure 2.1: A comparison of different aspects of traditional and advanced ceramics
[14]
ADVANCED MATERIALS TRADITIONAL MATERIALS
Raw materials
preparation
Forming
High temperature
processing
Finishing process
Characterization
Potters wheet
Slip casting
Flame kiln
Raw materials
Clay
Silica
Erosion
Glazing
Visible examination
Light microscopy
Chemically prepared powder
- Precipitation
- Spray dry
- Freeze dry
-Vapor phase
- Sol-gel
Slip casting
Injection molding
Sol-gel
HIPing
Rapid prototyping
Electron furnace
Hot press
Reaction sinter
Vapor deposition
Plasma spraying
Microwave furnace
Erosion
Laser machining
Plasma spraying
Ion implantation
Coating
Finishing
Light microscopy
X-Ray diffraction
electron microscopy
Neitron diffraction
7
2.1.1 Alumina (Al2O3)
Alumina is known as an alumina oxide or aluminum oxide which has chemical
formula Al2O3. Generally, alumina is formed by metal aluminium and occurs
naturally as the mineral corundum (Al2O3), diaspore (Al2O3.H2O), gibbsite
(Al2O3.3H2O) and most commonly as bauxite which is impure form of gibssite.
Alumina also known as many other names such alpha, gamma and delta regarding to
its nature and industry [15, 16]. Alumina is produced by using Bayer Process which
extracting alumina from bauxite.
The basic unit cell of aluminium oxide has two types of sites which are
hexagonal and octahedral. Hexagonal sites are the corner atom in the cell while the
octahedral sites are present between two layers of vertical stacking. Aluminium
cations are in 2/3 of the octahedral sites and oxygen anions are in 1/3 of the
octahedral sites. Each oxygen is shared between four octahedral sites permit strong
bonding and therefore give rise to the characterizations of the properties of alumina
[17].
Alumina is commonly available and is graded in different purities and it
typically graded into two main groups. The first of high alumina grades with atleast
99 % and the second alumina grades between 85% purity and going up to 99.9%
Al2O3 [18, 19]. These main groups can be further divided into subclasses according
to type, purity and intended services [20]. Typically, the selection of material
composition (purity) is important because each composition has a different percent
alumina content and influence of material properties. Basically, alumina with high
purity has the best mechanical properties with high density (>3.75g/cm3) and high
sintering temperature (1500ºC - 1900ºC) [20].
Generally, alumina is used either in pure form or as raw material mixed with
other oxide [4] and typically used as a base materials due to the high strength.
Alumina is the most cost effective and widely used materials in family of
engineering ceramics. The raw materials with high performance technical grade
ceramic is made readily available and reasonably priced, resulting in good value for
the cost fabricated alumina shapes. Alumina has unique combinations and useful for
electrical, mechanical and physicals properties [15]. Alumina is widely used for
engineering application such as ceramic, refractory, chemical industry, catalyst, filler
and glass industries due to its abundance and its multiple form as well as its
8
properties. The most and wide ranging use of alumina in the field of ceramics as an
insulating materials, electronic and mechacal ceramics[16].
Basically, the physical properties such as density, porosity and shrinkage are
important criteria that should be considered in the material selection process. Good
physical properties of the materials are very important to ensure that the material can
be used with optimum [3]. Mechanically, the alumina ceramic has great properties in
hardness, resistance to abrasive wear and dimensional stability. Basically, the
strength (hardness) properties of sintered alumina are strongly influenced by the
microstructure; porosity, grain size and pore size.
The excellent combination of mechanical and electrical properties such as
wear resistance and good hard makes extensive usage in the manufacturing industry
[15]. There are many ceramic processing methods can be used to produce a product
of the alumina with various sizes and shapes. Alumina can also be combined with
materials other metal or ceramic material such small amounts of silica, magnesia and
zirconia to obtain the desired properties suitable for the application.
2.1.2 Silica (SiO2)
The compound silica (SiO2) is typically formed from silicon and oxygen atoms [21].
A chemical compound is defined as a distinct and pure substance formed by the
union of two or more elements. Silica (SiO2) or silica dioxide is the most simple
silicate materials and it is obtained by in minerals, such as quartz and flint, and in
plants such as bamboo, rice and barley.
The three major forms of crystalline silica -quartz, tridymite and cristobalite-
are stable at different temperatures In its natural form it mostly occurs as a crystalline
phase and rarely in an amorphous state. Various phases may be formed, depending
on temperature, pressure and degree of hydration. At atmospheric pressure the
anhydrous crystalline silica may be classified to some phases according to the
temperature.
Silica is a hard, chemically inert and has a high melting point attribute to the
strength of the bond between the atoms. Basically, the silica widely used in industrial
development especially in glass, foundry and ceramic depending on their
characteristics. It is widely used because it is inexpensive, hard, chemically stable
and relatively infusible and has the ability to form glasses.
9
Figure 2.2: Relationships among the chemical term on silicon [22]
2.2 Fabrication of ceramic component
Basically, fabrication of ceramic component involves several stages which starts
from the formulation of the raw materials in the powder forms or particles till the
characterization of solid end products as shown in Figure 2.3.
Figure 2.3: Processing of the ceramic structure [3]
Silicon
(an element)
Plus oxygen Plus oxygen and other
elements
Silicon
(an element)
Silicates Silicates Silica
Raw materials
Preparation of the green body
(slurry casting or powder
preparation)
Drying process
Sintering / Firing process
Finishing
Characterizations
10
Typically, ceramic or particle powder is used as a basic in fabrication of the
ceramic body for producing green body and sintered body structure. This ceramic
powder or particles need to be characterized prior to the next processing stage of the
ceramic [4, 5, 23]. This is because starting powder always strongly influences the
ceramic green body as well as to the sintered body. Therefore, the characterizations
of the ceramic powder become greatest interest in ceramic formulation i.e. particle
size, powder agglomerate and exaggerated [4, 24, 25]. Previous studies in this field
of characteristic properties show that both of the particle distribution either narrow or
broader promote the densification rate at the low sintering temperature [12, 25-28].
In fact, most ceramic investigation consider particle geometry [24, 25] and behaviour
[4, 5] relates them to the end product of ceramic properties.
The next stage of the processing ceramic route is the forming stage where the
raw material of the ceramic is produced to desired size or shape before the
densification process. Usually, this can be done using several techniques such as
compaction, casting , extrusion and many more [3, 4, 15]. Compaction or pressing is
one of the common techniques that is commonly used to produce the green part of
ceramic. In this work, the pressing technique is applied in the fabrication of ceramic
alumina powder. The pressing technique is a process of compaction and shaping of a
dry and semidry powder or granule material in a rigid die or flexible by applying
sufficient pressure directly in vertical direction. In general, compaction (pressing)
processing consists of the three steps: filling of the die (mould), powder compaction
and ejection of the compacted powder and this technique is widely used in ceramic
processing due to the simple formation technique and gives accurate dimension [3].
However, some of the ceramic powder is difficult to compact especially for the very
fine and hard particles. Due to this condition, a small binder is needed during this
forming process to create a sticky surface so that particle can bind easily in
producing green body [4, 29]. For the example, polyvinyl alcohol (PVA) is
commonly used as a binder for ceramic oxides (as shown in Table 2.1) and also can
act as the lubricant of the ceramic compaction and it is always used to remove the
compacted green body from the mould. Furthermore, the lubricant is also important
in reducing distortion [30].
11
Figure 2.4: Schematic for powder compaction process [3]
Table 2.1: Additives used in Powder Pressing
Ceramic
powder
Additive
Binder Plasticizer Lubricant
Alumina Polyvinyl alcohol Polyethylene glycol Aluminiumstrearate
MnZn ferrite Polyvinyl alcohol Polyethylene glycol Zinc strearate
Barium titanate Polyvinyl alcohol Polyethylene glycol Strearic acid
Alumina substrate / Microcrystalline KOH : Wax, talc and clay
spark plasma wax emulsion Tatnic acid
Steatite insulator Microcrystalline Water Colloidal talc and wax
wax and clay
Refractories Ca /Na lingo sulfonate Water Stearic acid
Tile Clay Water Colloidal talc and clay
Then, the next stage of ceramic processing that is considered is the
consolidation stage. This is because the processing of ceramic components used
powder as particles that need to be dried and fired to form a hard and strong body.
Because the ceramic has properties of high melting temperature, inert and highly
sensitive to thermal physical changes. Therefore, this ceramic component needs to
be done in a controlled environment for producing good and quality of the end cast
products. In fact, there are many factors or parameters that can influence this
consolidation stage [24] which directly will influence the ceramic property. This will
be explained in detail in the following section.
Die fill stage Compaction Part ejection
Powder
Die
Lower punch
Upper punch
Green
compact
12
2.3 Drying on ceramic structure
In consolidation of ceramic powder to the green ceramic component, the drying
needs to be carried out prior to the next sintering stage. This process plays an
important role in ceramic forming operation to ensure the quality of structure is in
good condition. Basically, drying needs to be done before heating (firing) to make
sure free water is removed and avoid abrupt thermal changing which can cause steam
spallation occur to higher sintering temperature.
In ceramic processing, drying is one of the most important processes
especially for the ceramic porous material where the pore is filled with moisture or
liquid [8, 31]. Therefore, the drying needs to be conducted in order to remove free
water from the green body which brings the drastic changes in thermal gradient
during this stage and at the same time effects the sintered body structure at the final
stage of the processing. Drying of the ceramic materials involves complicated
mechanisms because normally it will exhibit the shrinkage. Therefore, most of the
ceramic must be dried as thoroughly as possible slowly and carefully to avoid failure
during the drying process [32].
2.3.1 Mechanism of drying in ceramic materials
Basically, drying is a process to remove moisture from the body by applying the heat
[33]. The drying process in most ceramic materials is very crucial since it can
influence the mechanical, physical and characteristic of the body. Typically, this
process is accompanied by physical and structural changes that are called shrinkage
mechanisms.
In drying, there are several physical mechanisms which contribute to
moisture migration such as moisture transport either by individual diffusion and
capillary flow or combined effect of moisture, temperature or pressure gradient [33].
In addition, drying materials are classified into hydroscopic and non hydroscopic
materials. For the hydroscopic materials, there are two types of water found in
porous materials like ceramic which are free water and bound water. Free water is
water filled in ceramic cell cavity and it is easy to evaporate while bound water is
referring to the water in a ceramic bonded to wall cell [7]. Typically, the bound water
13
is chemically attached to the cell wall and needs a large amount of energy to
evaporate.
Drying mechanisms involve the evaporation and migration of phase (liquid or
gas) from interior part to surface and this is a continuous process until to the body is
dried as shown in Figure 2.5. The drying consists of two parts; linear and non linear.
For the linear, water moves from the internal to the surface which involves the
capillary suction (free water) and non linear, the moisture is driven out from the pore
by diffusion (vapour phase) mechanism. All these movement and phase migration
relates to the heat, mass and gas transport between interior dried body and external
environment. This mechanism of water always depends on the hydroscopic nature
(water held to the particle) and properties of the materials, drying condition and the
way heat is applied (external or volumetric heating). As mentioned in the paragraph
above, two types of water found in ceramic and free water is usually evaporated first
as compared to the bound water at the beginning of the drying process. The free
water is fully removed during the drying process and it is slightly different to the
bound water which is removed by applying a large amount of heat.
Figure 2.5: Schematic of the drying model [7]
14
2.3.2 Drying technique
There are several methods of drying which are commonly used to remove water or
moisture such as conventional (room and oven) microwave and etc., every method
has their own characteristics and techniques to dry body structure (green body) and
this indirectly influences the final structure.
2.3.2.1 Room drying
Room drying is one of the most common drying methods and it is a traditional
method that is still widely used up till now. This procedure of drying strongly
depends on the outside environment [34]. In addition, drying is frequently a dual
process which involves evaporation and migration. Theoretically, conventional
drying involves the transportation of heat to the surface of material and then followed
by the movement of water from the heated body. However, in room drying , the
evaporation only acts immediately to the surface of the body and delayed action in
the migration of water from the internal structure (heated body) which is dependent
on the rate of diffusion through the materials [32-34]. One advantage of room drying
is low initial cost. However, there are many disadvantages for this method; low
energy efficiency and more time consuming heating processes especially for thick
materials [35, 36] to remove water from the heated body during the migration
process. In general, room drying is typically controlled by the condition of the
environment where it is difficult to predict the drying quality and speed of air.
Therefore, this condition can strongly give influence to the excess of the moisture
movement that will result to uncontrolled shrinkage mechanism. Furthermore, this
shrinkage mechanism gives high impact which leads to the crack formation and
finally brings to the failure of materials. Thus, imperfect heating can cause product
rejection and energy waste due to the uncontrolled parameter of the drying system.
Therefore, determining of the proper drying technique is needed in order to minimize
the microstructure defect and the processing time cycle.
15
2.3.2.2 Oven drying
Oven technique is a common laboratory drying method and is one of the alternatives
to drying that can reduce the processing time and cost of production. In general,
proper methods which can control the temperature is essential for quality of the
ceramic structure. This is because ceramic becomes weak when drying occurs
rapidly at high temperature. However, this drying technique is slightly different than
the room condition due to its potential to control every processing parameter by
setting the soaking heating time and also heating rate at the beginning stage [34].
Therefore, among the advantages of this drying is that it can dry products
successfully, and at the same time it can help can avoid the failure of the ceramic
body. Basically, the slow changes of temperature gradient will initiate the internal
diffusion process of pore water that brings to the slow motion of water to the dried
boundary layer. This mechanism needs a slow and controlled environment especially
for the pore section of brittle materials that are highly exposed to the shrinkage
problem. However, this drying technique also has its disadvantages where the heat
only acts on the surface than the internal will result in the removal of the surface
shrinks than the bottom. Furthermore, the oven is not an efficient drying technique
and also uses high energy to remove water from the internal structure during the
period of the drying process.
2.3.2.3 Microwave drying
Today, the microwave technique is one of the most widely used in drying method
approaches to improve the physical property materials because this method offers
much greater production outcomes and offers various heating conditions [37, 38].
Basically, microwave drying is capable to create higher drying rate and energy
efficiency than conventional heated air drying. Several works related to this
microwave drying technique mentioned that microwave is widely applied to various
kinds of raw materials and products including food, vegetables, fruits and wood as
well as ceramic materials [32, 39]. In fact, microwave is one of the most clean drying
technology as the usage of microwave heating can minimize the environment
emission problems via its internal and volumetric heating mechanism, and at the
same time can still produce better product quality [40]. Furthermore, this technique
16
can reduce the processing time because it allows the penetration of heat directly
through the internal of material structure where the volumetric heating drives the
water away from the interior, hotter section [41] and this technique is very important
for most of the ceramic body to remove water completely (as shown in Figure 2.6).
The drying process is important in ceramic body in order to remove water as much as
possible for preparing samples before firing at high temperature techniques [8]. Due
to the condition where heat can be generated throughout the volume of the materials
and resulting volumetric heating, the possibility in getting rapid heating and uniform
structure is high [8]. During microwave processes, the water in the material absorbs
microwaves throughout the entire mass causing molecular vibrations with respect to
the oscillating electric field of microwaves and thus heating simultaneously
throughout the material as indicated [38] .
Basically, the microwave heats materials internally within the materials
instead of originating from external heating sources and the depth of penetration of
the energy vary in different materials. During microwave processing, the microwave
energy penetrates through the materials and some of the energy is absorbed by the
materials and is converted to heat which can lead the interior part of the materials to
become hotter than its surface due to the interior part achieving higher temperature
and drying first; at the same time, it can reduce thermal stresses that cause cracking
during processing [41]. In addition, this internal heating mechanism also offers a
uniform energy absorption, heat and moisture distribution within the porous structure
such as ceramic [39].
Microwave processing of materials offers the potential for reducing
production time for ceramic materials due to the energy saving; and microwave
energy has proven to be an efficient and reliable form of heating for a wide range of
industrial processes [42]. Schroeder and Hackett [43] reported that a benefit from
using microwave heating in foundry in which the process time is reduced to one half
to one tenth of that required by conventional heating [37]. Therefore, microwave is
an appropriate heating device including the potential for reductions in manufacturing
costs and shorter processing time [44]. Table 2.2 shows the comparison of energy
saving of ceramic manufactured by the conventional drying and firing, and
microwave drying and firing. In contrast to conventional drying, heat generated by
microwave energy depends on microwave absorption efficiency of materials
including types and the dielectric properties of materials [45].
17
Figure 2.6: Heating pattern in conventional and microwave furnaces [41]
Table 2.2 : Comparison of energy savings for conventional and microwave drying
and firing of ceramics [41]
Energy savings (x106 kW -h/yr)
conventional
drying
microwave
drying
conventional
firing
microwave
firing
Total energy
saving
Brick and tile
56.1 28.05 198.9 19.9 207.06
Electrical
porcelain 3.52 1.76 12.48 1.25 12.99
Glaze 16.63 8.3 58.97 5.89 61.37
Pottery 1.96 0.98 6.94 0.69 7.23
Refractories 10.87 5.4 38.53 3.85 40.08
Sanitary ware 25.04 12.52 88.76 8.88 92.4
Advanced
ceramics 1.3 0.65 4.6 0.46 4.79
Total
(x106 kW -h/yr)
(x106 PJ/yr)
115.42
0.42
57.66
0.21
409.18
1.47
40.92
0.15
425.92
1.5
Conventional Microwave
Sample Heating
element
Insulation Metal shell Furnace Cavity
Microwave port
18
2.4 Sintering of ceramic structure
Most of the unfired dried ceramic bodies must go through the sintering process to
produce a hard and strong body by creating diffused particles and this is important
for the ceramic production [46]. Actually, sintering is the process to convert powder
to a dense product by bonding together the particles until it is adhere of each other
and this process is well known as the thermally activated process of the compact.
Generally, this process can be carried out at the temperature below the melting point
which ranges from 0.5 to 0.75 of the melting point [4] until full densification is
obtained. In fact, most of the ceramic bodies must be sintered to produce
microstructure with the required properties. Basically, sintering is a process where
the fully densification takes place and involves the development of new ceramic
structure [4]. Typically, sintering also influences the characteristic of the ceramic
structure and properties. This condition leads to the changes of ceramic performance
such as strength, shrinkage, porosity, grain size etc. The strength of ceramic structure
is increased due to the formation of diffused bonding between the particles [29]. This
will also lead to the formation of grain growth; sometimes the abnormal grain
growth. Typically, this complex process of sintering is influenced by a variety of
parameters and factors such as powder characteristics (particle size), distribution of
dopant addition and sintering conditions (temperature, applied pressure, time and
atmosphere). At the same time, the shrinkage mechanism takes place in the sintered
structure as the void spaces between the particles reduce when water starts to
eliminate. Another characteristic is increment of average grain size. Sintering is
divided into two basically categories: solid state sintering and liquid phase sintering.
19
Table 2.3: Selected ceramic composition and the sintering process used during
densification [29]
Composition Sintering process
Al2O3 Solid state sintering with MgO additive
Liquid phase sintering with silicates phase
MgO Liquid phase sintering with silicates phase
Si3N4 Liquid phase sintering with oxide additive (e.g Al2O3 and Y2O3) under
nitrogen gas pressure or under an externally applied pressure
SiC Solid state sintering with B and C additive
Liquid phase sintering with Al, B and C or oxide additive
ZnO Liquid phase sintering with B2O3 and other oxide additive
BaTiO3 Liquid phase sintering with TiO2 rich liquid
Pb(Zr,Ti)O3(PZT),
(Pb,La)(Zr,T)O3 (PLZT)
Sintering with a lead rich liquid phase: hot pressing
ZrO3/ (3-10 mol %) Solid state sintering
Mn-Zn and Ni-Zn ferrites Solid state sinterin under controlled oxygen atmosphere
Porcelain Vitrification
SiO2 gel Viscous sintering
2.4.1 Solid state sintering
In general, solid state sintering consists of three stages (as shown in Figure 2.7)
which is initial, intermediate and final [4, 5]. At the initial stage, the particles are in
contact with each other and developed the neck growth by diffusion mechanism.
Then, it is followed by the intermediate stage where pores shrink through the mass
transport by the neck growth. Once the grain growth begins, the pore phases are
intersected by grain boundaries and at the same time the microstructure can be
developed at the final stage. Basically, the pores shrink continuously and cause small
grains to be consolidated to form larger grains.
Figure 2
Sintering is typically accompanied by an increment o
system. The driving forces for sintering (solid state sintering) which consists of three
portions such as the curvature of the particle surface, an externally applied pressure
and chemical reaction gives rise to the free energy with reduction of surface energy
(as shown in Figure 2.8)
Step 1: neck formation
Initial state:
Packed powder particle
Pore
2.7: Schematic of three stages in sintering [4]
Sintering is typically accompanied by an increment of the free energy of a
The driving forces for sintering (solid state sintering) which consists of three
portions such as the curvature of the particle surface, an externally applied pressure
hemical reaction gives rise to the free energy with reduction of surface energy
)
Step 1: neck formation Step 3: grain growth
Initial state:
Packed powder particle Final state:
Grain boundaries and pore
Powder
particles
20
f the free energy of a
The driving forces for sintering (solid state sintering) which consists of three
portions such as the curvature of the particle surface, an externally applied pressure
hemical reaction gives rise to the free energy with reduction of surface energy
Step 3: grain growth
Grain boundaries and pore
21
.
Figure 2.8: Schematic of driving force for sintering [4].
In general, the reduction of the surface free energy is accompanied by the
densification process with producing shrinkage. This is due to the transport matter
from inside the grain into the pores and another process is coarsening where there is
no in producing the shrinkage [4]. Coarsening is a process of the rearrangement of
matter between different parts of the pore surface to produce the microstructure and
this process occurs simultaneously with the densification process, as shown in Figure
2.9. Basically, there are two characteristics of the solid state sintering which are
shrinkage component and porosity [4].
In general, the common difficulty in densification for the polycrystalline
(Al2O3) powder compact is it needs a high temperature during the sintering process
and at the same time increases the sintering rate. However, the excessive sintering
tends to cause the ceramic materials to become weak because of the porosity and
grain growth. This is because the coarsening process is strongly related to the grain
growth. The domination of coarsening phenomenon in solid state sintering typically
will hinder the production of dense body with high density. Also, the coarsening
process will promote an uncontrolled grain growth which is typically divided into
Pa
Applied pressure
Chemical reaction Surface free energy
Dense solid
22
two categories: normal and abnormal grain growth. Thus, controlling the grain
growth during the sintering process is one of the most important parts in the
formation of good ceramic structure because uncontrolled grain growth may lead to
the production of ceramic with undesirable properties such as low in strength and
cracking. The sintering aid was introduced to the ceramic system in order to enhance
the diffusion properties of the particles or improve diffusion by creating a small
amount of liquid phase during sintering. In addition, controlling the starting powder
is also needed in solid state sintering as the smaller particle size can increase the
sintering rate. Therefore, controlling powder is important in regard to reduce
sintering temperature and at the same time to produce the good properties of the
ceramic body [4, 24, 25].
Figure 2.9: Schematic of densification and coarsening process of
microstructure during firing process [4]
Grain
Grain
Densification
process
Coarsening
process
23
Figure 2.10: The surface of an Al2O3 ceramic during sintering [4, 29]
2.4.2 Liquid phase sintering
Another common process for densification of ceramic is sintering in the presence of
the reactive liquid (liquid phase sintering). This process is always used for ceramic
materials which are difficult to densify by solid state sintering in order to enhance
densification rate, achieve grain growth or produce specific grain boundary
properties. Furthermore, the liquid phase sintering is an effective densification
process to alumina ceramic by adding an additive which can reduce the sintering
temperature and processing time. Basically, the process of liquid phase sintering has
three sequential stages which are particle rearrangement, solution precipitation and
coalescence (as illustrated in Figure 2.11)[4].
24
Figure 2.11: Schematic evolution of powder compaction during liquid phase
sintering [4]
In the first stage, the liquid phase is formed with the particulate solid at the
sintering temperature. Basically, wetting liquid plays an important role to provide a
capillary force which can pull the solid particles together. Also, it can induce particle
rearrangement due to the influence of surface tension, giving more efficient packing
particle. This rearrangement of the particles can lead to faster densification with full
density being achieved in a liquid phase sintering compared to the solid state
sintering. Then, the densification proceeds to the next stage by a solid solution of the
solid materials at the contact point, diffusion through the liquid phase and the
precipitation at solid surface sites outside the contact area. Therefore, densification
occurs at a faster rate in liquid phase sintering due to the materials being transported
more rapidly in liquid than in solid. At the final stage of the liquid phase sintering is
solid state skeleton network. This process is slow because there are large diffusion
distances in the coarsening structure.
Sh
rin
ka
ge, ∆
L/L
o
Time
1
2
3
Initial Compaction
Rearrangement
Solution/
precipitation
Additive
Solids
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